1. Field of the Invention
The present invention relates to a projecting device, and more particularly, to a projecting device for displaying electronic images.
2. Description of the Prior Art
Please refer to FIG. 1. FIG. 1 is a schematic diagram of a projecting device 10 of an LCD projector according to the prior art. The projecting device 10 comprises a light source 12, a uniform illumination optical device 14, a light-separating device 16, a trichromatic prism 18, three modulating devices 20, a projecting lens 22 and three focussing lenses 17, 19 and 21. The light source 12 produces a white beam of light. The uniform illumination optic device 14 installed in front of the light source 12 converges the light emitted from the light source 12 into an approximately square-shaped beam with a uniformly distributed illumination. The light-separating device 16 separates the square-shaped beam into red (R), green (G) and blue (B) color input beams. The approximately square-shaped trichromatic prism 18, having three input sides 18a and an output side 18b, combines the red (R), green (G) and blue (B) input beams into an output beam. The three modulating devices 20 are installed in front of their associated input sides 18a of the trichromatic prism 18 to modulate the three input beams. The three focusing lenses, 17, 19 and 21, separately installed in front of the three modulating devices 20, focus the three input beams from the light-separating device 16 onto the three modulating devices 20. The projecting lens 22 installed in front of the output side 18b of the trichromatic prism 18 projects the combined output beam from the trichromatic prism 18 onto a screen 24. Each modulating device 20 consists of a transparent monochromatic liquid crystal display panel used to display a monochromatic image. The trichromatic prism 18 combines the three monochromatic images to form the output color image.
The light-separating device 16 comprises a first dichroic mirror 26 to separate the red light from the square-shaped white beam from the uniform illumination optic device 14. A reflecting mirror 27 reflects the red light from the first dichroic mirror 26 onto the focusing lens 17. The remaining light reflects from the dichroic mirror 26 to a second dichroic mirror 28 that separates out the blue light and reflects it to the focusing lens 19. Two optical lenses 30 and two reflecting mirrors 32 pass and reflect the remaining green light to the focusing lens 21.
FIG. 1 clearly shows that the distance traveled by the green light is much longer than that of the red and blue light. Since traveling distances affect light intensities, the two optical lenses 30 installed in front of the two reflecting mirrors 32 are essential to converge the green light so as to compensate for its loss of intensity.
However, the installation of the two optical lenses 30 makes the structure of the light-separating device 16 complicated and costly. It is therefore an important objective to provide a projecting device with a simple structure that can solve the problem of unequal traveling distances for the three color beams.